Darwin and son
In 1880, Darwin and son (Francis) published the findings of their Phototropism Experiment which using the tip of the coleoptile. Through this experiment, the existence of indole-3-acetic acid (IAA), which is a type of auxin was discovered. Research in the later years found that auxin is needed in stimulating cells growth (elongation) and involved in regulating plant growth.
In examples, low concentration of auxin accelerates root growth while high concentration of auxin inhibits the growth of buds and shoots. In fact, the indole-3-acetic acid, IAA is the best performing type of auxin and exist largely on the growing part of leaves.